net: phy: c45: add genphy_c45_an_config_eee_aneg() function

Add new genphy_c45_an_config_eee_aneg() function and replace some of
genphy_c45_write_eee_adv() calls. This will be needed by the next patch.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>